Korean researchers give silver nanoparticles the bends.

A new stretchy material conducts electricity about as well as bare metal and could be used to make large-scale stretchy electric circuits for displays, transistors, or lighting.

The biggest challenge for making stretchy electric circuits is maintaining conductivity when the material is pulled taut. One option for doing this is to load a block of elastic material with metal nanoparticles that can conduct electricity. But at the high concentrations of nanoparticles needed for good conductivity, the materials become brittle and easily broken. Those breaks then separate the nanoparticles and disrupt the conductivity of the material.

Unyong Jeong, at Yonsei University in Seoul; Jongjin Park, at the Samsung Advanced Institute of Technology; and their colleagues sidestepped this problem by spinning a stretchy material (made from a block of rubbery butadiene sandwiched between lengths of polystyrene) into fibrous mats. A two-step process filled the mats with silver nanoparticles to make the material conductive.

First the scientists soaked the fibers in a solution of silver ions. The solvent swelled the polymer fibers, allowing the silver ions to sneak inside the fibers. Then they chemically transformed those ions into silver nanoparticles. That created a stretchy mat that was both coated with silver nanoparticles on the outside and loaded with them on the inside its fibers.

The conductivity of an unstretched 150-micrometer thick mat is comparable to that of the thin strips of gold typically used for large-scale devices, Jeong says. The material maintains its conductivity (dropping about 40 percent to around 2,200 Siemens per centimeter) when fully strained.

The fibers are key to this behavior. At strains up to 40 percent, the pulling forces just straighten out the curves in the fibers without disrupting the contacts between the nanoparticles inside. The material still conducts at higher strains because the low volume of elastic fibers (2 micrometers wide) holds the nanoparticles close together.

The scientists used this material to build a stretchy radio-frequency antenna, tunable from 2.2 GHz to 1.0 GHz depending on the amount of stretch. They also made stretchy electrodes to light an LED, and wrapped their conductive material around a string, all with good conductivity.

In the future this material might be useful for stretchable transistors or lighting, Jeong says. And it would be easy to make stretchy circuits on a large scale because the scientists can print the silver solution (and thus control the final distribution of metal nanoparticles) using common industrial processes like nozzle or inkjet printing.

Reading the process reminded me of being a kid and stretching out thick rubber bands to their maximum and writing messages on them. Upon returning the rubber band to it's rest state, the message usually couldn't be read. I wonder if this has been tried with conductive inks. Granted it's most likely been tried and failed, but the memory hit while reading this and seemed interesting.